Fresh meat products, including poultry, are susceptible to contamination by microorganisms that contact meat surfaces immediately after slaughter and evisceration, where organisms in the intestinal tracts can be transferred to other organs during processing. Contaminating microorganisms include bacteria such as Salmonella and Campylobacter species, Listeria monocytogenes, Escherichia coli and other coliforms, and other enteric organisms. Once bacteria such as Salmonella contact tissue surfaces, they rapidly attach and are difficult to remove even with chlorine disinfectant permitted for use in poultry sprays and chill tanks. In beef processing, a particularly virulent strain of E. coli denoted 0157:E7 was reported to contaminate hamburger meat sold by a fast-food chain and to cause several deaths in the U.S. in 1993. Food poisoning from other organisms such as Listeria and Campylobacter or from spoiled meats in general are intermittently reported in the public press.
The problems created by Salmonella bacteria in poultry products are particularly noteworthy. Currently, Americans spend approximately $20 billion annually on poultry products, consuming about 80 pounds per capita. Approximately 35% to 45% of poultry reaching U.S. consumers is contaminated with Salmonella species. Improper cooking and physical transfer of the bacteria to food handling surfaces and thence to other foods result in the spread of the microorganisms, causing gastrointestinal disorders and, in some cases, death.
Breeders, hatcheries, feed ingredient suppliers, farms, processors, and distributors have all been implicated as contributors to Salmonellae contamination in chickens and turkeys (Villarreal, M. E., et al., J. of Food Protection 53: 465-467 (1990)). Contamination of but a few birds can lead to broader range contamination of other birds and cross-contamination to carcasses. Bacterial proliferation and other signs of spoilage can be delayed by refrigeration, but there is a limit to the degree of refrigeration that can be imposed on meat products, short of freezing the meat, and some bacteria such as psychrophiles can survive and even flourish at temperatures approaching the freezing point. It is thus preferable to control and destroy Salmonella and other microbial contaminants during processing to reduce the initial number of organisms on the meat.
Poultry processing is similar to the processing of other meat animals. Briefly summarized, caged birds arrive by truck at the processing plant. The birds are hung by their feet on shackles in a dressing line, stunned and bled via throat cuts. After bleeding and while still hung, the birds are scalded, plucked and transferred to an evisceration line, where they are manually or mechanically eviscerated, inspected and spray-washed. The spray may contain chlorine as a disinfecting agent. The last step of the process is chilling in a chill tank, by movement through a counterflow of cold water. The carcasses must reach an internal temperature of 5.degree. C. or below, which usually takes about 45 minutes to one hour in a typical many-thousand gallon tank. After reaching this temperature, the carcasses are packaged or further cut into parts, and refrigerated or frozen.
Salmonella and other organisms can survive the scalding process, which involve temperatures of about 50.degree. to 58.degree. C. Though cross-contamination can occur during any stage of processing, the major problems arise during and after evisceration when microorganisms are freed from the intestinal tract and transferred to other tissue surfaces. When carcasses are placed in the chill tank, organisms and unremoved viscera and visceral contents enter the water and can come in contact with other carcasses.
The U.S.D.A. and F.D.A. allow the use of chlorine in the water, up to 50 parts per million (ppm), to destroy some of these organisms. Upper range chlorine levels transfer to the air and can irritate factory workers, so lower levels, e.g., 20 ppm, are typically employed. This compromises antimicrobial effectiveness, as does organic matter and debris that accumulate in water and consume available chlorine. Indeed, even the upper allowable chlorine levels cannot eliminate or significantly reduce pathogenic organisms. In addition, chlorine in process waters has a tendency to react with a variety of organic materials, both from water and from poultry, to form a series of chloro-organic molecules, including species, e.g., trihalomethanes and chloramines, that have been implicated as mutagens and carcinogens.
Chlorine dioxide, which is less reactive with water components such as ammonia and nitrogen compounds, has been considered as an alternative disinfectant to chlorine in poultry processing. Chlorine dioxide can significantly reduce Salmonella and other unwanted microbial contaminants of meat surfaces, and at levels in water which are approximately one-seventh of that required for chlorine to achieve comparable effects.
Though chlorine dioxide has also been found to react with fewer amino acids than does chlorine (3 rather than 18), there is increasing evidence that the reactions cause undesirable effects on poultry surfaces. For example, it has been observed that chlorine dioxide, at the 1.4 ppm level in chiller water, was effective in reducing many bacteria and caused no detectable off-flavors on treated broilers, but the skin of the chickens was lighter in color than control carcasses, and the normal pinkish-white appearance had changed to grayish-white. Use of chlorine dioxide was curtailed in poultry processing as a result of sporadic retail complaints about "bleached" or old-looking carcasses. Moreover, subsequent chlorine dioxide experiments resulted in periodic episodes of severely discolored (blue-black) birds and random poor bacteriocidal efficacy.
Color changes occur from a combination of the oxidation of blood hemoglobin to methemoglobin, the oxidation of caretenoid colorants in poultry fat, and the reaction of chlorine dioxide with the amino acids tyrosine and tryptophan to form colored species. As a small molecule, chlorine dioxide can diffuse into tissues such as capillary walls and fat to effect oxidative changes. Similar color changes were noted by Villarreal, cited above, who studied the effect of slow-release chlorine dioxide levels on Salmonella levels in turkeys. While a major reduction in Salmonella organisms was observed, a bleaching of the skin was noted, especially on the wings and breasts of the carcasses. Skin areas like the neck exhibited a pale brownish color.
Irradiation has been approved by the U.S. government as an alternative antimicrobial treatment. However, irradiation appears to be not viable for most poultry processors due to the high capital plant cost, high operating costs, and the additional cost of transporting carcasses to such facilities. Irradiation may also pose occupational risks to poultry factory workers.
Therefore, it would be desirable to have an effective process for removing Salmonella and other unwanted microorganisms from meat carcasses such as poultry.